page_title: Ceramic matrix composite
section_title: Properties under tensile and bending loads, crack resistance
hierarchical_section_title: Ceramic matrix composite / Properties / Mechanical properties / Properties under tensile and bending loads, crack resistance
caption_attribution_description: Deutsch: Bild enthält Vergleichskurven verschiedener Verbundkeramiktypen und von SSiC in Single-Edge-Notch-Bend-(SENB)Versuchen. Der "Formale Spannungsintensitätsfaktor (SIF)" ist bei den Verbundkeramiken durch Normierung auf den Anfangskerb errechnet worden.
context_page_description: Ceramic matrix composites are a subgroup of composite materials as well as a subgroup of ceramics. They consist of ceramic fibers embedded in a ceramic matrix. Both the matrix and the fibers can consist of any ceramic material, whereby carbon and carbon fibers can also be considered a ceramic material.
context_section_description: The influence and quality of the fiber interface can be evaluated through mechanical properties. Measurements of the crack resistance were performed with notched specimens (see figure) in so-called single-edge-notch-bend (SENB) tests. In fracture mechanics, the measured data (force, geometry and crack surface) are normalized to yield the so-called stress intensity factor (SIF), KIc. Because of the complex crack surface (see figure at the top of this article) the real crack surface area can not be determined for CMC materials. The measurements, therefore, use the initial notch as the crack surface, yielding the formal SIF shown in the figure. This requires identical geometry for comparing different samples. The area under these curves thus gives a relative indication of the energy required to drive the crack tip through the sample (force times path length gives energy). The maxima indicate the load level necessary to propagate the crack through the sample. Compared to the sample of conventional SiSiC ceramic, two observations can be made:
  All tested CMC materials need up to several orders of magnitude more energy to propagate the crack through the material.
  The force required for crack propagation varies between different types of CMCs.
  In the table, CVI, LPI, and LSI denote the manufacturing process of the C/SiC-material. Data on the oxide CMC and SiSiC are taken from manufacturer datasheets. The tensile strength of SiSiC and Al
  2O
  3 were calculated from measurements of elongation to fracture and Young's modulus, since generally only bending strength data are available for those ceramics. Averaged values are given in the table, and significant differences, even within one manufacturing route, are possible.
  Tensile tests of CMCs usually show nonlinear stress-strain curves, which look as if the material deforms plastically. It is called quasi-plastic, because the effect is caused by the microcracks, which are formed and bridged with increasing load. Since the Young's modulus of the load-carrying fibers is generally lower than that of the matrix, the slope of the curve decreases with increasing load.
  Curves from bending tests look similar to those of the crack resistance measurements shown above.
  The following features are essential in evaluating bending and tensile data of CMCs:
  CMC materials with a low matrix content (down to zero) have a high tensile strength (close to the tensile strength of the fiber), but low bending strength.
  CMC materials with a low fiber content (down to zero) have a high bending strength (close to the strength of the monolithic ceramic), but no elongation beyond 0.05% under tensile load.
  The primary quality criterion for CMCs is crack resistance behavior or fracture toughness.
caption: Curves of toughness measurements of various ceramic composites and SiSiC[18]
  Legend: SiSiC: conventional SiSiC, SiCSiC(CVI) and CSiC(CVI): SiC/SiC and C/SiC manufactured in CVI processes, CSiC(95) und CSiC(93): C/SiC manufactured by the LPI-method, Ox(PP): oxide ceramic composite, CSiC(Si): C/SiC manufactured via the LSI process.
exif: {}